The calibrating of a magneto-inductive flow-measuring device is usually performed with water as a reference medium. Water is slightly diamagnetic, with the magnetic conductivity, or magnetic susceptibility, of water being relatively small. If the appropriately calibrated, flow-measuring device, or appropriately calibrated, device-type, is used at a later point in time, in the field, to measure flow of a medium having a high magnetic conductivity (such being the case, for example, when the medium has ferromagnetic fractions and, thus, a very high susceptibility), then the measured values are significantly corrupted, since the magnetically conductive medium influences the magnetic resistance of the measuring arrangement quite appreciably. The magnetically conductive medium can be e.g. water with magnetite additive. In the case of measurements of this type, measurement errors of up to 100% can occur. A reliable and reproducible measuring of the volume- or mass-flow is, in the case of such large measurement errors, naturally, no longer possible.